The present invention relates to sorbents and processes for removing pollutants from gas streams using such sorbents. More particularly, the sorbents of the present invention are resistant to physical degradation which results from recurring adsorption and regeneration. More specifically, the invention is directed to removing nitrogen oxides, sulfur oxides and hydrogen sulfide from gas streams.
The nitrogen oxides which are pollutants are nitric oxide (NO) and nitrogen dioxide (NO.sub.2 or N.sub.2 O.sub.4). The relatively inert nitric oxide is often only difficultly removed, relative to NO.sub.2. The lower oxide of nitrogen, N.sub.2 O (nitrous oxide), is not considered a pollutant at the levels usually found in ambient air, or as usually discharged from effluent sources. Nitrous oxide, however, degrades (decomposes) in the atmosphere to produce nitric oxide and thus eventually becomes a polluting component.
Sulfur oxides considered to be pollutants are sulfur dioxide and sulfur trioxide.
Particularly obnoxious sources of nitrogen and sulfur oxide pollutants are power plant stack gases, automobile exhaust gases, heating plant stack gases, and various industrial process effluents such as smelting operations and nitric and sulfuric acid plants.
Power plant emissions represent an especially formidable source of nitrogen oxides and sulfur oxides, by virtue of the very large tonnage of these pollutants in such emissions discharged into the atmosphere annually. Moreover, because of the low concentration of the pollutants in such emissions, typically 0.05% or less for nitrogen oxides and 0.3% or less for sulfur dioxide, their removal is difficult because very large volumes of gas must be treated.
Hydrogen sulfide is a pollutant in the effluents of the following operations: coal gasification, coal liquefaction, oil shale processing, tar sands processing, petroleum processing and geothermal energy utilization.
Of the few practical systems which have hitherto been proposed for the removal of nitrogen oxides from power plant flue gases, all have certain disadvantages. One such process entails scrubbing the gas with a slurry of magnesium hydroxide or carbonate; the slurry is regenerated by treatment with ammonia. This process, however, produces by-product ammonium nitrate which is difficult to dispose of, and also requires cooling and reheating of the flue gas stream.
Processes for the removal of nitrogen oxides from gases using various sorbents are discussed in the following: U.S. Pat. No. 2,684,283 to Ogg, Jr. et al (sorbent: mass of ferric oxide and sodium oxide); U.S. Pat. No. 3,382,033 to Kitagawa (sorbent: porous carrier impregnated with FeSO.sub.4 +H.sub.2 SO.sub.4, FeSO.sub.4, FeSO.sub.4.(NH).sub.4 SO.sub.4, PdSO.sub.4, KMnO.sub.4, KMnO.sub.4 +H.sub.2 SO.sub.4, KClO.sub.3, NaClO+NaOH, NaClO.sub.2 +NaOH, Na.sub.2 MoO.sub.4, K.sub.2 S.sub.2 O.sub.3, Na.sub.2 S.sub.2 O.sub.3 +NaOH, NaHPO.sub.4, Na.sub.2 O.sub.2, As.sub.2 O.sub.2 +NaOH, CuCl.sub.2, or ICI.sub.3 +NaOH); U.S. Pat. No. 3,498,743 to Kyllonen (use of a bed of finely divided solid sodium carbonate); and U.S. Pat. No. 3,864,450 to Takeyama et al (use of a catalyst consisting essentially of carbon impregnated with sodium or potassium hydroxide).
Various methods have been proposed for the removal of sulfur dioxide from power plant flue gases, but all of these have disadvantages. For example, wet scrubbing systems based on aqueous alkaline materials, such as solutions of sodium carbonate or sodium sulfite, or slurries of magnesia, lime or limestone, usually necessitate cooling the flue gas to about 55.degree. C. in order to establish a water phase. At these temperatures the treated gas requires reheating in order to develop enough buoyancy to obtain an adequate plume rise from the stack. Moreover, such processes create products involving a solid waste disposal problem.
Various solid phase processes for the removal of sulfur dioxide which have hitherto been proposed also have disadvantages. The use of limestone or dolomite, for example, to adsorb sulfur dioxide creates a waste disposal problem because the solid is not regenerated.
Processes for the removal of sulfur oxides from gases using various sorbents are discussed in the following: U.S. Pat. No. 2,992,884 to Bienstock et al (sorbent: alkali metal oxide dispersed on a carrier such as alumina or chromia); U.S. Pat. No. 3,411,865 to Pijpers et al (sorbent: alkali metal oxide and iron oxide dispersed on a carrier such as alumina, magnesia or chromia); U.S. Pat. Nos. 3,492,083 and 3,669,617 to Lowicki et al (sorbent: oxide, hydrated oxide or hydroxide of aluminium, zinc, iron or manganese and an oxide or hydroxide of an alkali metal or alkaline earth metal); U.S. Pat. No. 3,589,863 to Frevel (porous alkali metal bicarbonate aggregates); U.S. Pat. No. 3,755,535 to Naber (sorbent: activated alumina or magnesia impregnated on inert carrier); U.S. Pat. No. 3,948,809 to Norman et al (sorbent: bauxite and alkali metal carbonate); U.S. Pat. No. 3,959,952 to Naber et al (sorbent: alumina carrier impregnated with copper and aluminum, magnesium, titanium or zirconium) and United Kingdom 1,154,009 (sorbent: vanadium compound and an alkali metal compound).
U.S. Pat. No. 3,880,618 to McCrea et al concern the simultaneous removal of sulfur and nitrogen oxides from gases using alkalized alumina or alkali metal carbonate or oxide.
U.S. Pat. No. 4,071,436 to Blanton, Jr. et al describes the removal of sulfur oxides using reactive alumina.
Alkalized alumina is discussed in the following: D. Bienstock, J. H. Fields and J. G. Myers, "Process Development in Removing Sulfur Dioxide from Hot Flue Gases" 1. Bench-Scale Experimentation, Report of Investigations 5735, United States Department of the Interior, pp. 8-17; U.S. Pat. No. 3,551,093 to J. G. Myers et al and U.S. Pat. No. 3,557,025 to Emerson et al. As discussed hereinbelow in greater detail, alkalized alumina sorbents, heretofore utilized for flue gas treatment have exhibited severe degradation of their attrition resistance due to the chemical processes of adsorption and regeneration.
The alkalized alumina sorbent is manufactured by precipitating dawsonite (NaAl(OH).sub.2 CO.sub.3) from a solution of Al(SO.sub.4).sub.3 and Na.sub.2 CO.sub.3 at 90.degree. C. The resulting solid is then heated to 130.degree. C. to dry the residue moisture and crushed to a small size. Since the dawsonite is formed through precipitation, it has a very tight solid structure with little room to absorb SO.sub.2. Therefore, the chemically bonded H.sub.2 O and CO.sub.2 have to be removed through calcination at high temperatures in order to form a porous sorbent. ##STR1##
The calcinated sorbent (NaAlO.sub.2), known as alkalized alumina, is thereafter useable in a flue gas treatment process.
Sodium is an integral part of the whole crystal structure of alkalized alumina. The concentration of sodium in alkalized alumina is about 25% by weight.
The chemical process of adsorption produces changes in the sorbent and creates internal forces that cause sorbents of a type similar to those of the present invention, e.g., alkalized alumina sorbent, to attrite (crumble) rapidly. The sorbents of the present invention do not suffer from this attrition problem which has been associated with sorbents of a similar type, such as alkalized alumina.
As adsorption proceeds, the sulfite/sulfate product layer growth takes place in both directions from the initial pore boundary, however, the growth into the substrate material is limited to only a very thin layer for the impregnated sorbent. As the product layer grows into the alkalized alumina material itself, it disrupts and distorts the crystal structure. The product molecule (Na.sub.2 SO.sub.3 and Na.sub.2 SO.sub.4) volumes are much larger than the unreacted molecules (Na.sub.2 O) so the product layer produces a very disturbed and weakened material. As the growth continues, the product layer buckles and cracks producing pathways even deeper into the substrate body. The effect of this process is to create physical stresses that dramatically increase sorbent attrition. The growth proceeds with both impregnated and coprecipitated sorbent until all the sodium is consumed or until all the void space within the pore is occupied. Most of the surface area, and consequently the sodium, exists in the many very small pores of the impregnated sorbent. The dimension of these pores decreases continuously to sizes orders of magnitude smaller than the average pore diameter. In fact, many of the pores are of the size of the product molecule.
U.S. Pat. Nos. 4,323,544 and 4,426,365 both assigned to the assignee of the present invention, concern processes for the removal of nitrogen oxides using a sorbent comprising alumina having a surface area of about 20 m.sup.2 /g and an alkaline component comprising at least one salt of a Group IA (alkali metal) or Group IIA (alkaline earth metal).
As pointed out above, a major drawback of heretofore used sorbents for removal of sulfur oxides and/or nitrogen oxides is that such sorbents suffer from attrition. The sorbents of U.S. Pat. Nos. 4,323,544 4,426,365, which are quite effective in removing pollutants from waste gas streams, begin to suffer irreversible attrition at 175.degree. C. Accordingly, it would be quite advantageous to have a sorbent which is not only effective in removing gaseous pollutants such as sulfur oxides and nitrogen oxides, but is also able to withstand high temperatures without undergoing attrition.
The present invention provides sorbents that do not unduly degrade (does not unduly attrite) as a result of chemical use.
The present invention further provides a method of removing nitrogen oxides and, optionally, sulfur oxides, from waste gas streams simultaneously, in a single process. Moreover, in the present invention it is possible to treat waste gas streams at temperatures at which the streams still have adequate buoyancy to obtain good plume rise from the stack. The sorbents of this invention remove NO.sub.2, as well as the relatively inert NO, in an efficient manner.
The present invention also provides for the removal of nitrogen oxides and sulfur oxides from waste gases (which process produces elemental nitrogen and elemental sulfur) without producing solid waste product which would create a disposal problem. The process of the present invention utilizes only relatively small quantities of natural gas or other hydrocarbon fuel.
The present invention also provides for the removal of hydrogen sulfide.